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Contribution of calendar ageing modes in the performances degradation of supercapacitors during power cycling
Microelectronics Reliability 50 (2010) 1796–1803
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Contribution of calendar ageing modes in the performances degradation of supercapacitors during power cycling O. Briat *, J.-M. Vinassa, N. Bertrand, H. El Brouji, J.-Y. Delétage, E. Woirgard Laboratoire IMS, CNRS UMR 5218, IPB – Université Bordeaux 1, 351 Cours de la Libération, 33405 Talence Cedex, France
a r t i c l e
i n f o
Article history: Received 1 July 2010 Accepted 19 July 2010 Available online 11 August 2010
a b s t r a c t Due to their high specific power and their long cycle life, supercapacitors (SC) are very interesting devices for on-board energy applications such as hybrid-electric vehicles. However, in order to ensure optimal performances during the whole vehicle lifetime, the reliability of the SC must be quantified. Thus, accelerated ageing modes based on both calendar life tests and power cycling tests have been investigated and the performances degradation of the SC was quantified using periodic characterization tests based on impedance spectroscopy measurements. Hence, the aim of this paper is to determine the contribution of calendar ageing modes in the performances fading of supercapacitors during power cycling. The degradation of the performances was quantified through the monitoring of the parameters for a SC impedance model. Then, the results of calendar life tests were used to define an equivalent acceleration factor for power cycling by considering the impact of the voltage and the temperature individually. The obtained results confirm that the evolution of model parameters during power cycling corresponds to an equivalent calendar ageing, leading to a global approach for the study of the supercapacitors ageing. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction In micro-hybrid vehicles, the discontinuous power demand due to starting, acceleration and braking leads to repetitive charge/discharge current levels of about few hundreds amperes over a period of a few seconds for the supercapacitors module. On the other hand, this mission profile is also comprised of rest periods, from about a few tens of seconds in an urban driving cycle to a few hours or several days during vehicle parking. To ensure vehicle performances during its whole lifetime, the reliability of the supercapacitors (SC) must be quantified by accelerated ageing tests. Among these tests, both power cycling and calendar life tests are useful since they address the real operating modes previously described, even if this is not done in a separate way. Thus, the main goal of this paper is to quantify the part played by calendar ageing modes in the performances degradation of the supercapacitors during power cycling. In the first part of this paper, a SC impedance model is proposed. It is based on a physical description of both the porous electrode and the electrode–electrolyte interface [1]. The model parameters were identified by a characterization method based on impedance spectroscopy. Then, in the second part, the periodic characterization tests results are used to monitor the evolution of the model parameters during the calendar life tests. * Corresponding author. E-mail address: [email protected] (O. Briat). 0026-2714/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2010.07.118
The results from the power cycling tests are compared to the calendar life test results and the equivalent floating voltage value for power cycling is presented. Finally, the performances degradation is quantified for the SC modules under several experimental conditions. The link between the evolution of the model parameters and the equivalent floating voltage, which is representative of an acceleration factor in the calendar ageing mode, is highlighted. 2. SC impedance model The proposed impedance model for supercapacitors is based on the physical description of the current pathway through the cell. This model takes into account the porous nature of the electrodes and both the electric and ionic conductivities of materials and electrolyte respectively. In the investigated frequency range [10 mHz–100 Hz] the impedance model is described by Eq. (1):
Z ¼ Rs þ
Rel sinðc:p=2Þ cosðc:p=2Þ þ j: 3 x1c C x1c C
ð1Þ
The parameter Rs represents the resistance of the terminals, of the current collectors and of the separator. It is primarily impacted by the collector and the conductivity of the terminals, but also by the properties of the binding agent composing the electrode structure. The resistance Rel depends on the electrolyte conductivity and is linked to the mobility of the ions inside the porous structure [2,3]. The double layer capacitance which is proportional to the available surface area of the electrodes is represented by C for
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Fig. 1. Voltage and temperature dependency of the model parameters for a new SC (2600 F–2.5 V).
c = 0. The parameter c allows the pore size distribution and slow phenomena such as charge recovery and self-discharge to be taken into account [4]. In practice, as c value is low, the total capacitance of the cell is close to the C value. A specific procedure was validated for the identification of these four parameters [5]. We then focused on the dependency of both the voltage and the temperature of these parameters in steadystate. These parameters were identified in a temperature range of [40 °C, 60 °C] for a bias voltage of 2 V and on a voltage range of [0.5 V, 2.5 V] for a steady-state temperature of 20 °C. The corresponding results obtained for a new SC are illustrated in Fig. 1 and were used to determine polynomial laws implemented in the impedance model. These results show that the two resistive terms are quasi independent on the voltage but decrease significantly when the temperature rises. The capacitance C depends primarily on the voltage and is not affected by the temperature. Finally, the parameter c depends both on the voltage and temperature with stable regions around 1 V and 20 °C, respectively. The results of Fig. 1 allow to quantify the sensitivity of the model parameters to voltage and temperature and they highlight the fact that it is essential to have a meticulous characterization procedure to monitor parameters evolution during ageing. 3. Calendar life tests In practice, the study of the calendar ageing mode for the SC is based on tests where the samples are maintained at a constant voltage and temperature with a continuous monitoring of the leakage current. These calendar life tests are stopped periodically, in order to carry out characterization tests based on the impedance measurements [6]. In these conditions, specific ageing modes can be identified due to the absence of current flowing through the tested cells.
In our case, the calendar life tests were performed on several cells under various floating voltages and ambient temperatures. The main goal was to highlight the influence of voltage and temperature on the ageing rate and to determine the calendar ageing laws that could be implemented in the presented simulation model. In this aim, a specific procedure was used in order to identify the model parameters from the impedance measurements. The results of the impedance measurements made on a 2600 F–2.5 V cell during the calendar ageing tests at 2.5 V and 65 °C are illustrated in Fig. 2. At first, these results show a constant increase in the impedance real part over the entire investigated frequency range. This change corresponds to a horizontal shift in the Nyquist plan without deformation of the overall shape of the impedance. In particular, the slope of the impedance at low frequencies which is linked to the parameter c of the model remains constant. The equivalent capacitance calculated from the imaginary part of the impedance at low frequencies decreases continuously during the ageing test. In order to study the dependency of the ageing rate with voltage, four cells (2600 F–2.5 V) were tested at [2.5 V, 2.6 V, 2.7 V and 2.8 V] floating voltages and 65 °C. The periodic impedance measurements were then used to identify the four parameters of the impedance model. Their evolution during ageing tests is represented in Fig. 3. As shown in Fig. 3, the electrolyte resistance Rel remains constant during the first part of ageing. Therefore, during this phase, the electrolyte conductivity does not seem to be affected by the eventual presence of impurities that might result from electrolyte decomposition or redox reactions at the electrode–electrolyte interface [7]. After approximately 2000 h, an increase of Rel occurs with a rate that depends on the floating voltage. Concerning c, the shape of its evolution is close to that of Rel. As c is linked to the porosity and effective surface area of the elec-
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Fig. 2. Periodic impedance measurements of a 2600 F–2.5 V cell during calendar life tests at 2.5 V and 65 °C.
Fig. 3. Model parameters changes during calendar life tests with an ambient temperature of 65 °C.
O. Briat et al. / Microelectronics Reliability 50 (2010) 1796–1803
trodes, the second phase of ageing could be due to reactions located inside pores instead of in the bulk electrolyte volume. The results presented in Fig. 3, show that the resistance Rs increases slowly during the first part of the ageing tests and then increases more quickly after a time that depends on the applied floating voltage. For 200 mV above the rated voltage value, Rs reaches twice its initial value after approximately 2000 h at 65 °C. Regarding the capacitance, Fig. 3 illustrates a continuous decrease with a rate that seems to be independent of the floating voltage. It can be noticed that the two end-of-life criteria are not reached simultaneously. For instance, for a floating voltage of 2.7 V and 2.8 V, Rs doubled before the capacitance has decreases down to 80% of its initial value. For lower floating voltages, this tendency is inverted. Thus, the power and energy performances of the cells are not affected in the same way and they depend on the floating voltage during ageing. From these results, it is obvious that the increase in the voltage and temperature leads to an exponential acceleration of the internal electrochemical reactions responsible for the irreversible degradation of the performances [8]. The inflexion point observed on the evolution of Rel, Rs and c could be due to the decomposition of electrolyte leading to the generation of gases and an increase in the internal pressure. In fact, case deformation and electrolyte release have been observed a few times in several tested cells after the apparition of the inflexion point on the plot showing the evolution of the parameters.
4. Cycle life tests The SC power cycling tests were based on periodic charge/discharge current pulses separated by rest periods, as shown in Fig. 4 for the 300 A and 400 A current profiles. The aim was to determine if the induced self-heating could lead to different degradation processes than those observed for calendar life tests. As the duration of these tests is long, calendar ageing could also partly be responsible for the performances degradation of the SC. In order to study the impact of the peak value of the current for a given self-heating, these two profiles were characterized by the same RMS value of 180 A. In practice, a small difference is observed between the two self-heating. The main reason is that the spectrums of the two current profiles differ and lead to different Joule losses when they are applied to the SC impedance.
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Furthermore, as cell impedance varies during power cycling, the induced self-heating varies. Therefore, preliminary tests were undertaken in order to quantify the thermal impedance of a SC cell [9]. Then, the RMS value was settled on in order to induce a given self-heating at the beginning of the power cycling tests as well as to reach the maximum allowable cell temperature when the impedance real part will double. As for the calendar life tests, a periodic characterization procedure was used. Thus, the power cycling was periodically stopped in order to make impedance measurements. These results are illustrated in Fig. 5 for a 2600 F–2.5 V cell, up to 20,000 cycles. The results of Fig. 5 show that, the real part of the impedance is increased and the capacitance is decreased during power cycling. However, the increase of the impedance real part is more pronounced at low frequency and is not constant over the entire frequency range as seen in the calendar life tests. The Nyquist plot shows a change in the slope at low frequencies which is linked to c [5]. Changes in the model parameters are illustrated in Fig. 6 up to 25,000 cycles for three SC cells (A and B technology). The shapes of these changes differ from those observed in Fig. 3 for the calendar life tests. Indeed, Rs, Rel and c start to increase from the beginning of the power cycling while they remained quasi constant during the first phase of calendar ageing. Furthermore, these results show that the peak value of the current and consequently the flow of both the ionic and electronic charges have an impact on the parameters changes. One hypothesis to explain the observed performances degradation is that the large amount of ions that flow during charge and discharge affects the integrity of the porous electrode, leading to a decohesion of some carbon particles. This could explain the increase in both resistive terms and c which is related to the porous behavior of electrodes. The power cycling was prolonged up to 100,000 cycles with a rest phase at 25,000 and 60,000 cycles. The changes in the model parameters are illustrated in Fig. 7. These results highlight the regeneration phenomenon that occurs when the power cycling is stopped. The magnitude of the regeneration step increases with the SC ageing. This regeneration phenomenon is complex and is not detailed here. However, the adsorption/desorption process of electrolyte molecules at the porous electrode/electrolyte interface could be the main cause of this phenomenon [10].
Fig. 4. SC voltage response to the 300 A and 400 A charge/discharge current profiles used for the ageing tests.
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Fig. 5. Periodic impedance measurements of a 2600 F–2.5 V cell during power cycling tests.
Fig. 6. Model parameters changes during power cycling tests.
O. Briat et al. / Microelectronics Reliability 50 (2010) 1796–1803
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Fig. 7. Influence of the regeneration phenomenon on the changes in the model parameters.
5. Comparison between calendar life and power cycling results on individual cells The main goal of this section is to quantify the reversible and irreversible part of SC performances degradation by comparing of
the calendar life and power cycling results. To do this, we have focused on the normalized evolution of both Rs and C for two types of cells as illustrated in Fig. 8. The average temperature of the cells during power cycling increases continuously because the power losses rise due to the
Fig. 8. Comparison of model parameters evolution between calendar life tests and power cycling tests.
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Fig. 9. Voltage response of one cell of a module on one cycle and corresponding voltage acceleration factor.
increase of the impedance real part. A strict equivalence between the cells temperatures for the two ageing modes is complex. However, Fig. 1 shows that Rs and C are quasi independent of the temperature around 60 °C. Therefore, we have considered the average temperature over the entire power cycling tests. The regeneration phenomenon observed when power cycling is stopped is not observed in calendar life tests. In order to determine an overall ageing model for SC, it is important to quantify the amount of reversible/irreversible ageing in power cycling. In this aim, an equivalent voltage value for the cell in power cycling was defined. The determination of this value was based on the multiplication of each voltage value for the cycle by its relative acceleration factor deduced from the calendar life results. Thus, the equivalent voltage value is given by Eq. (2):
U cyc ¼ U n þ 0:1
logðAV Þ logðbv Þ
6. Power cycling on supercapacitor modules The power cycling of SC modules was done with a specific current profile based on the typical power requirements of a microhybrid vehicle. Each module was based on the series association of four SC cells. Five modules were tested with different values of Ucyc by settling the maximum charge voltage. The voltage response of one cell of a module and the corresponding evolution of the instantaneous voltage acceleration factor (av) are shown in Fig. 9 for one cycle. As shown in Fig. 9, only the voltage values near Un will have a significant impact on the accelerated ageing.
ð2Þ
where Un is the rated voltage of the cell, bv is the voltage acceleration factor for 100 mV increase and AV is the average value of the instantaneous acceleration factors av which are defined by Eq. (3):
av ¼ ½bv
UU n 0:1
ð3Þ
From calendar life point of view, power cycling is equivalent to an equivalent floating voltage of the cell at Ucyc. In order to verify this approach on a significant number of cells, power cycling tests were performed on several supercapacitor modules.
Fig. 10. Direct and corrected equivalent voltage values for each supercapacitors module.
Fig. 11. Relative capacitance change vs. Ucyc (corrected) for each module after 90,000 cycles.
O. Briat et al. / Microelectronics Reliability 50 (2010) 1796–1803
Then, according to the results of the calendar life tests a correction factor was introduced in the calculation of Ucyc in order to take into account the temperature differences between the modules. This correction factor was defined for a temperature reference of 50 °C. Therefore, as shown in Fig. 10, the Ucyc value of the modules 4 and 5 was increased as their average temperature was 58 °C. In order to quantify the ageing, we have focused on the relative capacitance change of each module vs. the corrected value of Ucyc after 90,000 cycles, as illustrated in Fig. 11. Finally, Fig. 11 shows that the capacitance change is linked to Ucyc which can be considered as a quantity equivalent to a combined voltage and temperature stress level in calendar ageing mode. These first results can be used to define an ageing model of SC used in power cycling applications. 7. Conclusion In this paper, the contribution of calendar ageing modes to the performances degradation of supercapacitors during power cycling was evaluated. The proposed approach consisted in quantifying the reversible and the irreversible part of ageing during power cycling. In this aim, the impedance model parameters were identified and monitored during both calendar life and power cycling ageing tests using a periodic characterization procedure. Then, in order to compare the two ageing modes, an equivalent floating voltage for the power cycling mode was defined. This equivalent voltage was used to determine an equivalent acceleration factor of the ageing based on calendar life results. This approach was applied to several SC
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modules with different experimental conditions. Finally, the link between the equivalent floating voltage and the measured module capacitance which reflects the real performances degradation of SC has been highlighted. References [1] Conway BE. Electrochemical supercapacitors scientific fundamentals and technological applications. Kluwer Academic/Plenum Publisher; 1999. ISBN: 0-306-45736-9. [2] Lajnef W, Vinassa JM, Briat O, Azzopardi S, Woirgard E. Characterization methods and modelling of ultracapacitors for use as peak power sources. J Power Sources 2007;168(2):553–60. [3] Pandolfo AG, Hollenkamp AF. Carbon properties and their role in supercapacitors. J Power Sources 2006;157(1):11–27. [4] Jang JH, Yoon S, Ka BH, Jung Y, Oh SM. Complex capacitance analysis. J Electrochem Soc 2005;152(4):A1418–22. [5] El Brouji EH, Briat O, Vinassa JM, Henry H, Woirgard E. Analysis of the dynamic behavior changes of supercapacitors during calendar life test under several voltages and temperatures conditions. Microelectron Reliab J 2009;49:1391–7. [6] Kurzweil P. Ac impedance spectroscopy – a powerful tool for the characterization of materials and electrochemical power sources. In: 14th International seminar on double layer capacitors, USA; 2004. [7] Azaïs Philippe, Duclaux L, Florian P, Massiot D, Lillo-Rodenas MA, LinaresSolano A, et al. Causes of supercapacitors ageing in organic electrolyte. J Power Sources 2007;171(2):1046–53. [8] Bohlen O, Kowal J, Sauer DU. Ageing behaviour of electrochemical double layer capacitors. J Power Sources 2007;172(1):468–75. [9] Briat O, Lajnef W, Vinassa JM, Woirgard E. Power cycling tests for accelerated ageing of ultracapacitors. Microelectron Reliab J 2006;46:1445–50. [10] Lajnef W, Vinassa JM, Briat O, El Brouji H, Azzopardi S, Woirgard E. Quantification of ageing of ultracapacitors during cycling tests with current profile characteristics of hybrid and electric vehicles applications. IET Electr Power Appl 2007;1:683–9.
Contents lists available at ScienceDirect
Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel
Contribution of calendar ageing modes in the performances degradation of supercapacitors during power cycling O. Briat *, J.-M. Vinassa, N. Bertrand, H. El Brouji, J.-Y. Delétage, E. Woirgard Laboratoire IMS, CNRS UMR 5218, IPB – Université Bordeaux 1, 351 Cours de la Libération, 33405 Talence Cedex, France
a r t i c l e
i n f o
Article history: Received 1 July 2010 Accepted 19 July 2010 Available online 11 August 2010
a b s t r a c t Due to their high specific power and their long cycle life, supercapacitors (SC) are very interesting devices for on-board energy applications such as hybrid-electric vehicles. However, in order to ensure optimal performances during the whole vehicle lifetime, the reliability of the SC must be quantified. Thus, accelerated ageing modes based on both calendar life tests and power cycling tests have been investigated and the performances degradation of the SC was quantified using periodic characterization tests based on impedance spectroscopy measurements. Hence, the aim of this paper is to determine the contribution of calendar ageing modes in the performances fading of supercapacitors during power cycling. The degradation of the performances was quantified through the monitoring of the parameters for a SC impedance model. Then, the results of calendar life tests were used to define an equivalent acceleration factor for power cycling by considering the impact of the voltage and the temperature individually. The obtained results confirm that the evolution of model parameters during power cycling corresponds to an equivalent calendar ageing, leading to a global approach for the study of the supercapacitors ageing. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction In micro-hybrid vehicles, the discontinuous power demand due to starting, acceleration and braking leads to repetitive charge/discharge current levels of about few hundreds amperes over a period of a few seconds for the supercapacitors module. On the other hand, this mission profile is also comprised of rest periods, from about a few tens of seconds in an urban driving cycle to a few hours or several days during vehicle parking. To ensure vehicle performances during its whole lifetime, the reliability of the supercapacitors (SC) must be quantified by accelerated ageing tests. Among these tests, both power cycling and calendar life tests are useful since they address the real operating modes previously described, even if this is not done in a separate way. Thus, the main goal of this paper is to quantify the part played by calendar ageing modes in the performances degradation of the supercapacitors during power cycling. In the first part of this paper, a SC impedance model is proposed. It is based on a physical description of both the porous electrode and the electrode–electrolyte interface [1]. The model parameters were identified by a characterization method based on impedance spectroscopy. Then, in the second part, the periodic characterization tests results are used to monitor the evolution of the model parameters during the calendar life tests. * Corresponding author. E-mail address: [email protected] (O. Briat). 0026-2714/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2010.07.118
The results from the power cycling tests are compared to the calendar life test results and the equivalent floating voltage value for power cycling is presented. Finally, the performances degradation is quantified for the SC modules under several experimental conditions. The link between the evolution of the model parameters and the equivalent floating voltage, which is representative of an acceleration factor in the calendar ageing mode, is highlighted. 2. SC impedance model The proposed impedance model for supercapacitors is based on the physical description of the current pathway through the cell. This model takes into account the porous nature of the electrodes and both the electric and ionic conductivities of materials and electrolyte respectively. In the investigated frequency range [10 mHz–100 Hz] the impedance model is described by Eq. (1):
Z ¼ Rs þ
Rel sinðc:p=2Þ cosðc:p=2Þ þ j: 3 x1c C x1c C
ð1Þ
The parameter Rs represents the resistance of the terminals, of the current collectors and of the separator. It is primarily impacted by the collector and the conductivity of the terminals, but also by the properties of the binding agent composing the electrode structure. The resistance Rel depends on the electrolyte conductivity and is linked to the mobility of the ions inside the porous structure [2,3]. The double layer capacitance which is proportional to the available surface area of the electrodes is represented by C for
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Fig. 1. Voltage and temperature dependency of the model parameters for a new SC (2600 F–2.5 V).
c = 0. The parameter c allows the pore size distribution and slow phenomena such as charge recovery and self-discharge to be taken into account [4]. In practice, as c value is low, the total capacitance of the cell is close to the C value. A specific procedure was validated for the identification of these four parameters [5]. We then focused on the dependency of both the voltage and the temperature of these parameters in steadystate. These parameters were identified in a temperature range of [40 °C, 60 °C] for a bias voltage of 2 V and on a voltage range of [0.5 V, 2.5 V] for a steady-state temperature of 20 °C. The corresponding results obtained for a new SC are illustrated in Fig. 1 and were used to determine polynomial laws implemented in the impedance model. These results show that the two resistive terms are quasi independent on the voltage but decrease significantly when the temperature rises. The capacitance C depends primarily on the voltage and is not affected by the temperature. Finally, the parameter c depends both on the voltage and temperature with stable regions around 1 V and 20 °C, respectively. The results of Fig. 1 allow to quantify the sensitivity of the model parameters to voltage and temperature and they highlight the fact that it is essential to have a meticulous characterization procedure to monitor parameters evolution during ageing. 3. Calendar life tests In practice, the study of the calendar ageing mode for the SC is based on tests where the samples are maintained at a constant voltage and temperature with a continuous monitoring of the leakage current. These calendar life tests are stopped periodically, in order to carry out characterization tests based on the impedance measurements [6]. In these conditions, specific ageing modes can be identified due to the absence of current flowing through the tested cells.
In our case, the calendar life tests were performed on several cells under various floating voltages and ambient temperatures. The main goal was to highlight the influence of voltage and temperature on the ageing rate and to determine the calendar ageing laws that could be implemented in the presented simulation model. In this aim, a specific procedure was used in order to identify the model parameters from the impedance measurements. The results of the impedance measurements made on a 2600 F–2.5 V cell during the calendar ageing tests at 2.5 V and 65 °C are illustrated in Fig. 2. At first, these results show a constant increase in the impedance real part over the entire investigated frequency range. This change corresponds to a horizontal shift in the Nyquist plan without deformation of the overall shape of the impedance. In particular, the slope of the impedance at low frequencies which is linked to the parameter c of the model remains constant. The equivalent capacitance calculated from the imaginary part of the impedance at low frequencies decreases continuously during the ageing test. In order to study the dependency of the ageing rate with voltage, four cells (2600 F–2.5 V) were tested at [2.5 V, 2.6 V, 2.7 V and 2.8 V] floating voltages and 65 °C. The periodic impedance measurements were then used to identify the four parameters of the impedance model. Their evolution during ageing tests is represented in Fig. 3. As shown in Fig. 3, the electrolyte resistance Rel remains constant during the first part of ageing. Therefore, during this phase, the electrolyte conductivity does not seem to be affected by the eventual presence of impurities that might result from electrolyte decomposition or redox reactions at the electrode–electrolyte interface [7]. After approximately 2000 h, an increase of Rel occurs with a rate that depends on the floating voltage. Concerning c, the shape of its evolution is close to that of Rel. As c is linked to the porosity and effective surface area of the elec-
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Fig. 2. Periodic impedance measurements of a 2600 F–2.5 V cell during calendar life tests at 2.5 V and 65 °C.
Fig. 3. Model parameters changes during calendar life tests with an ambient temperature of 65 °C.
O. Briat et al. / Microelectronics Reliability 50 (2010) 1796–1803
trodes, the second phase of ageing could be due to reactions located inside pores instead of in the bulk electrolyte volume. The results presented in Fig. 3, show that the resistance Rs increases slowly during the first part of the ageing tests and then increases more quickly after a time that depends on the applied floating voltage. For 200 mV above the rated voltage value, Rs reaches twice its initial value after approximately 2000 h at 65 °C. Regarding the capacitance, Fig. 3 illustrates a continuous decrease with a rate that seems to be independent of the floating voltage. It can be noticed that the two end-of-life criteria are not reached simultaneously. For instance, for a floating voltage of 2.7 V and 2.8 V, Rs doubled before the capacitance has decreases down to 80% of its initial value. For lower floating voltages, this tendency is inverted. Thus, the power and energy performances of the cells are not affected in the same way and they depend on the floating voltage during ageing. From these results, it is obvious that the increase in the voltage and temperature leads to an exponential acceleration of the internal electrochemical reactions responsible for the irreversible degradation of the performances [8]. The inflexion point observed on the evolution of Rel, Rs and c could be due to the decomposition of electrolyte leading to the generation of gases and an increase in the internal pressure. In fact, case deformation and electrolyte release have been observed a few times in several tested cells after the apparition of the inflexion point on the plot showing the evolution of the parameters.
4. Cycle life tests The SC power cycling tests were based on periodic charge/discharge current pulses separated by rest periods, as shown in Fig. 4 for the 300 A and 400 A current profiles. The aim was to determine if the induced self-heating could lead to different degradation processes than those observed for calendar life tests. As the duration of these tests is long, calendar ageing could also partly be responsible for the performances degradation of the SC. In order to study the impact of the peak value of the current for a given self-heating, these two profiles were characterized by the same RMS value of 180 A. In practice, a small difference is observed between the two self-heating. The main reason is that the spectrums of the two current profiles differ and lead to different Joule losses when they are applied to the SC impedance.
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Furthermore, as cell impedance varies during power cycling, the induced self-heating varies. Therefore, preliminary tests were undertaken in order to quantify the thermal impedance of a SC cell [9]. Then, the RMS value was settled on in order to induce a given self-heating at the beginning of the power cycling tests as well as to reach the maximum allowable cell temperature when the impedance real part will double. As for the calendar life tests, a periodic characterization procedure was used. Thus, the power cycling was periodically stopped in order to make impedance measurements. These results are illustrated in Fig. 5 for a 2600 F–2.5 V cell, up to 20,000 cycles. The results of Fig. 5 show that, the real part of the impedance is increased and the capacitance is decreased during power cycling. However, the increase of the impedance real part is more pronounced at low frequency and is not constant over the entire frequency range as seen in the calendar life tests. The Nyquist plot shows a change in the slope at low frequencies which is linked to c [5]. Changes in the model parameters are illustrated in Fig. 6 up to 25,000 cycles for three SC cells (A and B technology). The shapes of these changes differ from those observed in Fig. 3 for the calendar life tests. Indeed, Rs, Rel and c start to increase from the beginning of the power cycling while they remained quasi constant during the first phase of calendar ageing. Furthermore, these results show that the peak value of the current and consequently the flow of both the ionic and electronic charges have an impact on the parameters changes. One hypothesis to explain the observed performances degradation is that the large amount of ions that flow during charge and discharge affects the integrity of the porous electrode, leading to a decohesion of some carbon particles. This could explain the increase in both resistive terms and c which is related to the porous behavior of electrodes. The power cycling was prolonged up to 100,000 cycles with a rest phase at 25,000 and 60,000 cycles. The changes in the model parameters are illustrated in Fig. 7. These results highlight the regeneration phenomenon that occurs when the power cycling is stopped. The magnitude of the regeneration step increases with the SC ageing. This regeneration phenomenon is complex and is not detailed here. However, the adsorption/desorption process of electrolyte molecules at the porous electrode/electrolyte interface could be the main cause of this phenomenon [10].
Fig. 4. SC voltage response to the 300 A and 400 A charge/discharge current profiles used for the ageing tests.
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Fig. 5. Periodic impedance measurements of a 2600 F–2.5 V cell during power cycling tests.
Fig. 6. Model parameters changes during power cycling tests.
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Fig. 7. Influence of the regeneration phenomenon on the changes in the model parameters.
5. Comparison between calendar life and power cycling results on individual cells The main goal of this section is to quantify the reversible and irreversible part of SC performances degradation by comparing of
the calendar life and power cycling results. To do this, we have focused on the normalized evolution of both Rs and C for two types of cells as illustrated in Fig. 8. The average temperature of the cells during power cycling increases continuously because the power losses rise due to the
Fig. 8. Comparison of model parameters evolution between calendar life tests and power cycling tests.
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Fig. 9. Voltage response of one cell of a module on one cycle and corresponding voltage acceleration factor.
increase of the impedance real part. A strict equivalence between the cells temperatures for the two ageing modes is complex. However, Fig. 1 shows that Rs and C are quasi independent of the temperature around 60 °C. Therefore, we have considered the average temperature over the entire power cycling tests. The regeneration phenomenon observed when power cycling is stopped is not observed in calendar life tests. In order to determine an overall ageing model for SC, it is important to quantify the amount of reversible/irreversible ageing in power cycling. In this aim, an equivalent voltage value for the cell in power cycling was defined. The determination of this value was based on the multiplication of each voltage value for the cycle by its relative acceleration factor deduced from the calendar life results. Thus, the equivalent voltage value is given by Eq. (2):
U cyc ¼ U n þ 0:1
logðAV Þ logðbv Þ
6. Power cycling on supercapacitor modules The power cycling of SC modules was done with a specific current profile based on the typical power requirements of a microhybrid vehicle. Each module was based on the series association of four SC cells. Five modules were tested with different values of Ucyc by settling the maximum charge voltage. The voltage response of one cell of a module and the corresponding evolution of the instantaneous voltage acceleration factor (av) are shown in Fig. 9 for one cycle. As shown in Fig. 9, only the voltage values near Un will have a significant impact on the accelerated ageing.
ð2Þ
where Un is the rated voltage of the cell, bv is the voltage acceleration factor for 100 mV increase and AV is the average value of the instantaneous acceleration factors av which are defined by Eq. (3):
av ¼ ½bv
UU n 0:1
ð3Þ
From calendar life point of view, power cycling is equivalent to an equivalent floating voltage of the cell at Ucyc. In order to verify this approach on a significant number of cells, power cycling tests were performed on several supercapacitor modules.
Fig. 10. Direct and corrected equivalent voltage values for each supercapacitors module.
Fig. 11. Relative capacitance change vs. Ucyc (corrected) for each module after 90,000 cycles.
O. Briat et al. / Microelectronics Reliability 50 (2010) 1796–1803
Then, according to the results of the calendar life tests a correction factor was introduced in the calculation of Ucyc in order to take into account the temperature differences between the modules. This correction factor was defined for a temperature reference of 50 °C. Therefore, as shown in Fig. 10, the Ucyc value of the modules 4 and 5 was increased as their average temperature was 58 °C. In order to quantify the ageing, we have focused on the relative capacitance change of each module vs. the corrected value of Ucyc after 90,000 cycles, as illustrated in Fig. 11. Finally, Fig. 11 shows that the capacitance change is linked to Ucyc which can be considered as a quantity equivalent to a combined voltage and temperature stress level in calendar ageing mode. These first results can be used to define an ageing model of SC used in power cycling applications. 7. Conclusion In this paper, the contribution of calendar ageing modes to the performances degradation of supercapacitors during power cycling was evaluated. The proposed approach consisted in quantifying the reversible and the irreversible part of ageing during power cycling. In this aim, the impedance model parameters were identified and monitored during both calendar life and power cycling ageing tests using a periodic characterization procedure. Then, in order to compare the two ageing modes, an equivalent floating voltage for the power cycling mode was defined. This equivalent voltage was used to determine an equivalent acceleration factor of the ageing based on calendar life results. This approach was applied to several SC
1803
modules with different experimental conditions. Finally, the link between the equivalent floating voltage and the measured module capacitance which reflects the real performances degradation of SC has been highlighted. References [1] Conway BE. Electrochemical supercapacitors scientific fundamentals and technological applications. Kluwer Academic/Plenum Publisher; 1999. ISBN: 0-306-45736-9. [2] Lajnef W, Vinassa JM, Briat O, Azzopardi S, Woirgard E. Characterization methods and modelling of ultracapacitors for use as peak power sources. J Power Sources 2007;168(2):553–60. [3] Pandolfo AG, Hollenkamp AF. Carbon properties and their role in supercapacitors. J Power Sources 2006;157(1):11–27. [4] Jang JH, Yoon S, Ka BH, Jung Y, Oh SM. Complex capacitance analysis. J Electrochem Soc 2005;152(4):A1418–22. [5] El Brouji EH, Briat O, Vinassa JM, Henry H, Woirgard E. Analysis of the dynamic behavior changes of supercapacitors during calendar life test under several voltages and temperatures conditions. Microelectron Reliab J 2009;49:1391–7. [6] Kurzweil P. Ac impedance spectroscopy – a powerful tool for the characterization of materials and electrochemical power sources. In: 14th International seminar on double layer capacitors, USA; 2004. [7] Azaïs Philippe, Duclaux L, Florian P, Massiot D, Lillo-Rodenas MA, LinaresSolano A, et al. Causes of supercapacitors ageing in organic electrolyte. J Power Sources 2007;171(2):1046–53. [8] Bohlen O, Kowal J, Sauer DU. Ageing behaviour of electrochemical double layer capacitors. J Power Sources 2007;172(1):468–75. [9] Briat O, Lajnef W, Vinassa JM, Woirgard E. Power cycling tests for accelerated ageing of ultracapacitors. Microelectron Reliab J 2006;46:1445–50. [10] Lajnef W, Vinassa JM, Briat O, El Brouji H, Azzopardi S, Woirgard E. Quantification of ageing of ultracapacitors during cycling tests with current profile characteristics of hybrid and electric vehicles applications. IET Electr Power Appl 2007;1:683–9.